The monitoring of gases and of aerosol concentrations, particularly of lubricating oil mist in working chambers of internal combustion engines, of fuel high pressure units for supplying internal combustion engines, or of power transmission gears, is of considerable importance in order to avoid damages. A rapid increase of the oil mist concentration is indicative of damages, for example of the tearing off of a lubricating film. As a result of frictional heat thus formed, oil vapors are formed which condensate to oil mist in the working chamber and thus result in a rapid increase of the oil mist concentration. If the resulting hazard is quickly recognized, then explosions and a concomitant threat to persons and further damages to the machine can be prevented with appropriate counteractions such as shutting down of the machine or of individual components of the machine. However, it is also possible to study specific gas components in such a working atmosphere by means of other sensors.
Furthermore, in addition to the tearing off of the lubricating film, so-called blow-throughs may occur between the piston and the associated cylinder wall in bearings of piston engines due to damaged piston rings, which cause a total damage of the piston/cylinder aggregate (“piston seizure”). An increase of the oil mist density with simultaneous rise in temperature due to the hot combustion gases is indicative of such blow-throughs.
First approaches for measuring the oil mist concentration are known from EP-A-0 071 391. In EP-A-0 071 391 it is suggested to draw the aerosol from the working chamber through a measuring compartment by means of a blower and to carry out therein a reflection measurement by means of a radiation source and a radiation sensor. The winged wheel blower suggested therein is intended for use with a plurality of compartments arranged parallel to each other.
The disadvantages of such an arrangement were already shown in WO-A-98/11331. In addition to the considerable constructive and operating requirements of such arrangement, the use of a blower for drawing out has been found to be insufficient, meaning that such a solution should be avoided. Moreover, the drawing out operation also draws waste air through the pipe system, and thus oil deposits in the form of oil bags can form which clog the pipelines and thus render the operation of the measuring device difficult or impossible.
In contrast thereto, in DD-A-239 474 and in GB-A-2 166 232 it is suggested to arrange, for each working chamber of the driving unit of an internal combustion engine, a sensor unit directly in the interior of the respective working chamber and to connect it via an optical or electrical transmission path to an evaluation unit arranged outside of the internal combustion engine. However, such a solution is associated with the disadvantage that the base concentration of oil mist and splash oil in the long run contaminates the sensors and, therefore, also leads to false alarms.
In contrast, in the already mentioned WO-A-98/11331 it is suggested to provide, in each working chamber to be monitored, a sensor unit with an extraction based on a venturi nozzle. Such a measuring device operates without mechanically moved parts and is, therefore, almost wearless. Something similar is also known from JP 7/310519 A. From this, and previously already from DE 26 08 390 A1, it is also already known that an optical measuring path that comprises an optical emitter and an optical receiver may be provided with a so-called curtain by means of which the above mentioned optical elements are kept as free as possible from oil mist deposits etc. This has been previously achieved in such manner that a fresh air flow was generated perpendicular to the optical measuring path in each case in the vicinity of the optical elements. However, this is complex in comparison with the effect.
Moreover, it has been found to be a disadvantage of the device according to WO-A-98/11331A that the energy expense of the venturi pump for drawing in the aerosol mixture from the driving chamber through the optical measuring path is very high, particularly when the optical measuring path is arranged in a casing that is connected with the venturi pump via a pipeline.